Besides minor language corrections, this tries in particular to
provide some clarity about the `step` setting in the Prometheus range
query and the `$__interval` variable:

- First of all: They are both always the same.

- What's really confusing (and arguably really un-helpful) is that the
  _Min time interval_ is multiplied by the resolution
  factor. Therefore, I just state the fact here now but don't add
  anything like "This should be the scrape interval" because it is not
  true (if resolution is `1/10`, a meaningful _Min time interval_ is
  generally a tenth of the scrape interval).

- More helpful is the _Min step_ parameter, because it is not
  multiplied by the resolution factor (which I have now stated
  explicitly). Of course, the _Min time interval_ has to be set to
  something small so that the _Min step_ parameter can kick in.

It would be good if we could also override the _Min time interval_
tool tip because that is where most people will look, and the current,
very Graphite-centric tool tip is very confusing for the Prometheus
query editor, in particular because it doesn't have any hint about the
resolution factor being applied. See #21413.

Signed-off-by: beorn7 <beorn@grafana.com>